The present invention relates to a liquid crystal display and more particularly to a liquid crystal display which is operable under relatively low voltage and at room temperatures.
At the present time liquid crystal displays are receiving considerable research and commercial attention since they present, in some circumstances, advantages compared to other types of electro-optical displays. For example, compared to cathode ray tubes, the electro-optical displays utilizing liquid crystal materials do not emit heat, may be of lower cost, and have a relatively long life. Further advantages are no wash-out under bright illumination and low power consumption.
It has been suggested that liquid crystal displays may be used in a wide variety of applications, some of these applications being specified in U.S. Pat. No. 3,322,485 to Williams, issued May 30, 1967. For example, such electro-optical displays may be used in portable electronic calculators and as the time indications for horological movements. For example, it has been suggested that an accurate electronic watch may be produced using a quartz crystal oscillator as its time frequency, a series of countdown circuits, and an electro-optical display consistng of a dial having a liquid crystal material. The dial may either present digital numerals, for example, made up of segments which are either activated or not activated, or alternatively may consist of segments which approximate, and appear to be similar to, the continuous circular movement of hands.
One difficulty which has been experienced with the previously suggested liquid crystal displays has been that their power consumption was relatively high. Those displays having a high power consumption are unsuitable for use in portable instruments using a self-contained battery. Another difficulty with certain of the previously suggested displays has been that they do not operate at all the temperatures which the devices may encounter, for example, at room temperatures or outside in a cold climate at below the freezing point. Still another difficulty has been with the clarity of the displays, that is, that the contrast compared to the background has not been sufficient under all ambient lighting conditions for readily ascertaining which segments have been activated. Still another--and in many respects the major--difficulty has been the relatively high voltge which has been required to operate the liquid crystal displays. In many cases the voltages required exceeded 12 volts (rms) and in still other cases they are greater than 5 volts (rms). Such a requirement necessitates the provision within the instrument of a plurality of batteries or special electronic circuits in order to raise the voltage to the required level.
Certain of th previously suggested liquid crystal displays used dynamic scattering effect and others the field effect (twister effect). Those using dynamic scattering consume power but very little power is comsumed in the displays, such as the displays of the present invention, which use the field effect.
In dynamic scattering mode a thin film of a nematic liquid crystal is positioned between two plates having aligned conductive segments. The liquid crystal molecules are aligned uniformly by use of alignment dopants so that its "off" state is optically clear. The segments are connected to a power source so that an electric field is applied across that portion of the liquid crystal which is between the segments. Above a certain threshold potential, ions which are present as impurities or can be added intentionally in the liquid crystal, undergo violent vertical movement exerting effective shearing force to the ordered array of the nematic liquid. The resultant abrupt local changes in the refractive indices of the liquid crystal causes the intense light scattering.
The field effect uses the same type of structure, that is, a thin layer of liquid crystalline material between two conductors. The liquid crystalline material used in field effect displays is different from those used in dynamic scattering displays. The field effect materials consist of rod shaped molecules wherein the dielectric constant parallel to the major molecular axis is considerably larger than that of the perpendicular; that is called positive dielectric anisotropy. In the field effect displays, surfaces of the two conductive plates are modified unidirectionally by appropriate physical and/or chemical means. Then the two plates are face to face in such a manner that the unidirectional characteristics of the two plates assume 90.degree.. Field effect materials enclosed in such an environment will become a unique optical medium, which can rotate the incident plane polarized light by 90.degree..